Modern processes in the beaded channels of steppe rivers of the Khoper-Buzuluk plain

Beaded-shaped channels (or chain-of-ponds), characterized by alternating lake-like extensions and narrow runs, are widespread on small rivers of the steppe zone of Russia. Extensions of channels – beads (pools or ponds), with a depth of up to 5–6 m, are of great importance for cattle breeding, providing a watering place during the dry periods of the year, when the flow of small rivers stops. However, the mechanisms of their specific channel shape formation are still debatable: it is explained by uneven siltation of channels, formation of whirlpools, groundwater discharge, karst processes or relict thermokarst. The aim of the study was to assess the role of water flow in the formation and support of the beaded shape of river channels. The tasks were to reveal the hydrological regime of the Kardail and Kupava rivers, determine the hydrodynamics of the flow in beads and runs, describe the characteristics of erosion and accumulation processes and the sediment transport, as well as to identify relict elements of the channel and floodplain structure that do not correspond to modern water state. Water flow of the studied rivers shows a long-term decrease in snowmelt flood, as evidenced by the rare flooding of the high-level floodplain, low rates of bank dynamics, and the discrepancy between the size of meanders and the width of the channel. The annual sediment load of the studied beaded rivers is low, estimated at 6,05 t/km2 in 2023. The reason is a low supply of sediments from the catchment area and low rates of erosion of channel bed and banks built of clay and silt. The present-day dynamics of water flow do not help the deepening and expansion of the beads. The flow reaches maximum speed (up to 1–1,3 m/s) in channel runs, while in channel extensions flow speeds drop to 0–0,3 m/s, supporting the sediment accumulation, despite the presence of whirlpools. A significant part of channel extensions is located on sites of former pools of the ancient meandering channel, inherited by the modern watercourse. On the former riffles the channel narrowed and a low-level floodplain was formed. Deep channel extensions were preserved due to the low sediment load and its fine composition, which prevents its rapid settling from the suspension. The results of the study suggest possible contribution of other factors to the formation of original deep pools, including the cryogenic ones.

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